Italy said on Monday it was very concerned over what it called an "unacceptable" decision by Indian authorities to jail two Italian marines who had been held in a police-guarded guest house for allegedly shooting two fishermen.

The Kerala police detained Massimiliano Latorre and Salvatore Girone in the southern Indian coastal town of Kochi on Feb 20, igniting a diplomatic dispute that has escalated.

The two marines were assigned to protect the Italian merchant vessel Enrica Lexie, which was sailing off the Indian coast when they opened fire to avert what they thought was a pirate attack.

Italy's foreign ministry director-general Giampiero Massolo spoke to the Indian Embassy's charge d'affaires, Saurabh Kumar, to express "very real concern" about the decision to transfer the two to the prison in Trivandrum, a ministry statement said.

The Italian government considered "such measures unacceptable given the status of the servicemen" and asked that the two sailors be transferred to a more "adequate" structure, it said.

Italy insists that India has no jurisdiction over the incident, which occurred in international waters.

Italy's foreign minister Giulio Terzi argues that, since the marines were aboard an Italian vessel, any investigation or trial should be conducted by Italy according to longstanding international rules governing the high seas.

Italy began assigning military teams to protect its merchant vessels in the Indian Ocean last year after a series of attacks by Somali pirates on Italian ships.

Pirates operating in small fishing vessels and fast motorboats have hijacked dozens of ships in the region in recent years, extracting millions of dollars in ransom.